This invention relates generally to gas turbine combustors and more particularly concerns reducing combustion instabilities in dry low NO.sub.x gas turbine combustors.
Gas turbines generally include a compressor, one or more combustors, a fuel injection system and a turbine. Typically, the compressor pressurizes inlet air which is then reverse flowed to the combustors where it is used to provide air for the combustion process and also to cool the combustors. In a multi-combustor system, the combustors are located about the periphery of the gas turbine, and a transition duct connects the outlet end of each combustor with the inlet end of the turbine to deliver the hot products of combustion to the turbine.
Gas turbine combustors are being developed which employ lean premixed combustion to reduce emissions of gases such as NO.sub.x. One such combustor comprises a plurality of burners attached to a single combustion chamber. Each burner includes a flow tube with a centrally-disposed fuel nozzle comprising a center hub which supports fuel injectors and swirl vanes. During operation, fuel is injected through the fuel injectors and mixes with the swirling air in the flow tube, and a flame is produced at the exit of the burner. The combustion flame is stabilized by a combination of bluffbody recirculation behind the center hub and swirl-induced recirculation. Because of the lean stoichiometry, lean premixed combustion achieves lower flame temperatures and thus produces lower NO.sub.x emissions.
These premixed systems are susceptible to combustion instabilities in the form of strong, unsteady pressure oscillations in the main combustion chamber. The oscillations are believed to be caused by the turbulent nature of the combustion process and the large volumetric energy release within the closed cavities of the combustor. If not suppressed, the combustion instabilities will severely limit the operating range of the combustor and may even lead to fatigue failure of combustor hardware. Acoustic, energy-absorbing liners are conventionally used to suppress combustion oscillations. However, these liners require cooling air injection which is typically not available in low NO.sub.x combustors. Moreover, acoustic liners are costly and allow air leakage which may have an adverse impact on combustor performance and emission levels. Another method of reducing combustion oscillations involves spreading out the heat release, i.e., decoupling the heat release from the pressure antinode. However, this method is not compatible with low NO.sub.x combustors because the flame must be kept short and compact for low CO emissions.
Accordingly, there is a need to enhance the stability and operating range of premixed low NO.sub.x gas turbine combustors by reducing or eliminating high frequency combustion instabilities.